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Corrects skipping of every other file, and transition from bytes back into header.

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This commit is contained in:
Thomas Harte 2017-12-23 19:20:04 -05:00
parent f4d414d6e4
commit 9a7e974579
1 changed files with 29 additions and 4 deletions
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33
Storage/Tape/Formats/CAS.cpp
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@ -78,6 +78,10 @@ CAS::CAS(const char *file_name) {
}
}
/*!
Treating @c buffer as a sliding lookahead, shifts it @c quantity elements to the left and
populates the new empty area to the right from @c file.
*/
void CAS::get_next(Storage::FileHolder &file, uint8_t (&buffer)[8], std::size_t quantity) {
assert(quantity <= 8);
@ -103,7 +107,10 @@ void CAS::virtual_reset() {
Tape::Pulse CAS::virtual_get_next_pulse() {
Pulse pulse;
pulse.length.clock_rate = 4800;
pulse.length.clock_rate = 9600;
// Clock rate is four times the baud rate (of 2400), because the quickest thing that might need
// to be communicated is a '1', which is two cycles at the baud rate, i.e. four events:
// high, low, high, low.
// If this is a gap, then that terminates a file. If this is already the end
// of the file then perpetual gaps await.
@ -128,10 +135,16 @@ Tape::Pulse CAS::virtual_get_next_pulse() {
default: break;
case Phase::Header: {
// In the header, all bits are 1s, so let the default value stand. Just check whether the
// header is ended and, if so, move on to bytes.
distance_into_bit_++;
if(distance_into_bit_ == 2) {
distance_into_phase_++;
if(distance_into_phase_ == (chunk_pointer_ ? 15360 : 3840)) {
distance_into_bit_ = 0;
// This code always produces a 2400 baud signal; so use the appropriate Red Book-supplied
// constants to check whether the header has come to an end.
if(distance_into_phase_ == (chunk_pointer_ ? 7936 : 31744)) {
phase_ = Phase::Bytes;
distance_into_phase_ = 0;
distance_into_bit_ = 0;
@ -140,7 +153,8 @@ Tape::Pulse CAS::virtual_get_next_pulse() {
} break;
case Phase::Bytes: {
int byte_value = files_[file_pointer_].chunks[chunk_pointer_][distance_into_phase_ / 11];
// Provide bits with a single '0' start bit and two '1' stop bits.
uint8_t byte_value = files_[file_pointer_].chunks[chunk_pointer_][distance_into_phase_ / 11];
int bit_offset = distance_into_phase_ % 11;
switch(bit_offset) {
case 0: bit = 0; break;
@ -149,6 +163,15 @@ Tape::Pulse CAS::virtual_get_next_pulse() {
case 10: bit = 1; break;
}
// Lots of branches below, to the effect that:
//
// if bit is finished, and if all bytes in chunk have been posted then:
//
// - if this is the final chunk in the file then, if there are further files switch to a gap.
// Otherwise note end of file.
//
// - otherwise, roll onto the next header.
//
distance_into_bit_++;
if(distance_into_bit_ == (bit ? 4 : 2)) {
distance_into_bit_ = 0;
@ -158,14 +181,16 @@ Tape::Pulse CAS::virtual_get_next_pulse() {
chunk_pointer_++;
if(chunk_pointer_ == files_[file_pointer_].chunks.size()) {
chunk_pointer_ = 0;
file_pointer_++;
phase_ = (chunk_pointer_ == files_.size()) ? Phase::EndOfFile : Phase::Gap;
} else {
phase_ = Phase::Header;
}
}
}
} break;
}
// A '1' is encoded with twice the frequency of a '0'.
pulse.length.length = static_cast<unsigned int>(2 - bit);
pulse.type = (distance_into_bit_ & 1) ? Pulse::Type::High : Pulse::Type::Low;